This invention relates to a method of determining whether radioactive contaminants are inside or outside a structure, using a detector located outside the structure.
When it becomes necessary for any reason to decontaminate radioactive piping or enclosures, it is necessary to remove radioactive contaminants from wherever they may be. If a structure such as a pipe is found to emit gamma rays that are characteristic of a particular contaminant, it is not always evident whether the contaminant adheres to the inside or the outside of the pipe or, if both, what the relative amounts are. This presents a problem because different techniques are used for cleaning the insides and the outsides of pipes, tubes or other such enclosing structures. One method used in the past for such cleaning operations has been to measure the radioactivity before cleaning, to clean or attempt to clean the outside of the structure, and to make a second measurement of radioactivity so as to determine whether the inside needs to be cleaned as well. This represents a wasted effort if all the measured radioactivity or a substantial portion of it came from radioactive contaminants contained inside the pipe, fittings or valves. Also, inadequate cleaning of the outside of the structure leads to measurements that give a false indication of internal contamination.
Even the method described above may be ineffective unless it is possible to clean or decontaminate a sufficiently large part of the outside portion of a structure in the vicinity of a measuring site to reduce the interfering effect of stray external radiation. In addition, there are conditions under which the difficulty of access to portions of a reactor during diassembly may make it difficult or impossible to decontaminate enough area around the measuring site to avoid the masking effect of radiation from surrounding external surfaces in view of the fact that radiation passing through a tube or valve is attenuated by the material of the tube or valve.
It is an object of the present invention to provide a better method of measuring the relative quantities of radioactive contaminants inside and outside a structure.
It is a further object of the present invention to provide a method for measuring the relative amounts of radioactive contaminants present inside and outside a structure without performing a preliminary decontamination.
Other objects will become apparent in the course of a detailed description of the invention.